Patent GB-921308-A (GB)
Title: Process for producing polymeric tubular films
IPC: B29C48/10
Filed: 1961-08-22
Abstract: Oriented tubular film of thermoplastic polymeric material is produced by extruding the material in the form of a tubular film in its formative state, advancing the tubular film at a predetermined initial rate through a zone in which it is quenched to a temperature below its formative state, and preferably to a temperature at least 20 DEG C. below the orientation temperature range, maintaining sufficient gaseous pressure within the tubular film to prevent collapse and to provide, when the tubular film is subsequently heated to a temperature within its orientation temperature range, an expansion to a diameter of at least twice the original diameter of the tubular film, advancing the tubular film through a zone in which it is heated to a temperature between 70 and preferably 80% of the lowest temperature of the orientation temperature range in degrees Centigrade and just below the orientation temperature range of the tubular film material, therafter advancing the tubular film through a zone in which its inner surface is heated by a source of radiant heat within the tubular film to a temperature within the orientation temperature range to <PICT:0921308/IV(a)/1> expand the tubular film to a diameter at least twice its original diameter, advancing, while expanding, the heated tubular film at a rate at least twice the initial rate of advancement, and cooling the tubular film while maintaining it substantially at its expanded diameter.  The final advancing rate may be up to 10 times the initial rate, and the ratio of the final to the initial advancing rate is preferably equal to the ratio of the final diameter to the extruded diameter of the tubular film.  As shown in the drawing, the polymeric material is melted in the extruder 10 and extruded through the annular die 11 to form a tubular film 14 which is drawn over but out of contact with the cooling or quench mandrel 12 by means of driven rubber squeeze rolls 13 which may be substituted by travelling endless belts.  The mandrel 12 is a hollow metal cylinder thermally insulated from the die by a non-metallic disc 15 having openings 26 round its periphery communicating with the atmosphere through conduit 27.  Cooling water is circulated through coils 30 disposed adjacent to the interior surface of the mandrel by means of inlet and outlet tubes 28 and 29 passing through the conduit 27.  The solidified tubular film is then drawn through an external guide ring 16 by means of a pair of nip rolls 24 and through the initial heating zone 19 comprising hot air blowers 17 and the electric radiant heater 18. Thereafter the film is drawn over an internal guide ring 23 and is heated internally to a temperature within the orientation temperature range by the pencil-type electric radiant heater 22 mounted on the guide ring 23 and connected to a power source by leads passing through the conduit 27.  The film when reaching this temperature is inflated by air or other gas admitted through inlet 20 and vented through outlet 21. The inflated and longitudinally stretched tubular film is finally cooled by drawing it through a cooling ring 25 which may contain cooling water coils.  In examples the polymeric materials used are respectively polypropylene of density 0,906, linear polypropylene of density 0,902, linear polyethylene of density 0,954, a 75/25 mixture of polyethylenes of densities 0,915 and 0,958 respectively, and a composition containing 92% polyvinyl chloride.  Films produced according to the invention do not display gauge variations magnified by the inflation of the film, and polypropylene and other crystalline polymers when used in the process do not suffer from "line drawing". Specification 695,567 and U.S.A.  Specification 2,987,765 are referred to.